Fog has covered the tri-state for most of this week because of a thick layer of a moist, mild, Atlantic airmass with very little wind to mix clouds out. Although fog is common this time of the year, there are a few things people don't realize about fog.

Fog is low lying clouds that reduce visibility, but it's actually only officially considered fog when visibility is below a certain number.

According to the American Meteorological Society, visibility needs to be reduced to less than 1 km (that's 0.62 miles) for fog to be officially declared. Otherwise it's simply considered "reduced visibility." When fog is expected to reduce visibility down to 0.25 miles or less, the National Weather Service will issue a dense fog advisory.

Unless your eyes are closed, visibility is never truly zero, but 0 visibility sometimes will show up on our maps. Our maps will round visibility readings down to the nearest tenth of a mile. If the current visibility report is less than 0.1 miles, it will read as zero.

Visibility this low translates to very poor driving conditions. 0.1 miles of visibility means fog will make anything less than 500 feet in front of you difficult to see. For some perspective, that's about half the length of a cruise ship, or less than five street lights.

During foggy conditions, drivers should never use their high beams. The reflection from the light will bounce off the water droplets and make it even more difficult for you and for approaching drivers to see. Leave plenty of leading distance between you and other drivers and drive under the speed limit. Fog will usually collect in areas near the water and improve inland and on busier streets.